Bulk density control of coking coal



1956 c, c. THROOP ETAL 2,765,266

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United States Patent 6 BULK DENSITY CGNTROL OF COKING COAL Charles C. Throop and Orien V. Wince, Monessen, Pa., assignors, by mesne assignments, to Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware Application June 8, 1953, Serial No. 360,174

4 Claims. (Cl. 202-25) This invention relates to a method and apparatus for controlling variations in the bulk density of coking coal due to variations in its surface moisture.

it is now generally recognized that one of the most important factors affecting the uniformity of coke oven operations and the quality of the coke produced is the bulk density of the coal that is charged into the oven. Changes in that bulk density not only cause irregular heating, which is reflected in the impaired quality of the coke; but also cause variations in oven capacity, which affect the coke yield. The principal cause of bulk density changes in coking coal is the variation in its surface moisture. As the surface moisture increases up to a limit of around 8% or 9%, the coal increases in volume and its bulk density decreases. Conversely, as this wet coal dries out, it shrinks in volume and its bulk density increases. Between dry coal and wet coal, the bulk density may vary as much as 15% or more; and in coke oven operations it is seldom uniform or predictable, since coking coals are generally a blend of different coals, some of which may have been freshly mined and have a low moisture content, and others of which may have more or less surface moisture, depending on the processing to which they have been subjected and on the weather conditions under which they have been transported and stored. While it is generally recognized that some moisture is desirable in the coke oven charge, the difficulty has been to control it, and keep it uniform.

It has heretofore been discovered that small amounts of oil added to wet coal cause a significant increase in its bulk density, as described in Patent No. 2,378,420 for Regulating the Bulk Density of Coke Charges, issued on June 19, 1941. Since that discovery, various methods and devices have been used with indifferent success to control the bulk density by adding varying amounts of oil and/or water to coking coals, ranging from dry coal to wet coal, in order to obtain and maintain the minimum moisture content and the specific bulk density that is best suited for a particular coking operation. Accordingly, the general object of the present invention is to provide a method and apparatus for controlling the bulk density of coking coals by the addition of oil and/ or water that will be an improvement over those now used.

In accordance with this invention, pulverized coking coal is delivered at a substantially uniform rate by weight onto a conveyor that is moving at a constant speed, whereby variations in the bulk density of the coal will be indicated by variations in the height of the coal on the moving conveyor, and adding to the coal before it is delivered to the conveyor predetermined proportions of at least one of the group consisting of water and oil in order to maintain the coal an the conveyor at a predetermined height corresponding to the desired minimum moisture content and to the desired bulk density.

The preferred embodiment of this invention is illustrated in the accompanying drawings, in which Fig. 1 is a schematic view of apparatus embodying this invention; and

2,765,266 Patented Oct. 2, 1956 Fig. 2 is a cross section of a portion of the apparatus shown in Fig. l, the plane of View being indicated by the line IIlI on Fig. 1.

In a coke oven plant, the raw coal as it comes directly from the mine, or from storage bins, or from the stockyard is generally delivered to a plurality of blending bins, each bin containing a difierent type of coal, or mixture of coals, that are to be blended in desired proportions. Two such bins, A and B, are shown in Fig. 1. At the outlet of each bin, there is provided a weight-controlling feeder 1 of conventional design for withdrawing a specified weight of coal per unit of time from the bin. Each feeder includes a conveyor 2, a weighing means 3 which supports a portion of the conveyor, a weight indicator 4, and an adjustable gate 5, at the outlet of the feeder. The gate is controlled by the weighing means and varies the amount of coal that is withdrawn from the bin, so that a specified weight of coal per unit of time will be discharged by the couveyor 3 to a hammer mill 6. Each feeder will, of course, discharge coal from its associated bin at a predetermined rate that will determine the composition of the final blend.

The hammer mill 6 is also of conventional design and includes a hopper 7, rotating hammers 8, grate bars E and a discharge chute ill. Coal is fed into the hammer mill from the feeders at a rate not exceeding the output capacity of the mill, so that the pulverized coal will be discharged from the mill at substantially the same rate as the raw coal is introduced therein, i. e., the pulverized coal will leave the mill at a substantially uniform rate by weight. Below the chute it) is mounted a delivery conveyor 15, preferably having an arcuate cross section (as shown in Fig. 2) which conveys the pulverized coal from the hammer mill to a larry car (not shown) for charging into the coke ovens. This conveyor moves at a uniform speed.

Since the pulverized coal is discharged from the hammer mill onto the conveyor 15 at a substantially uniform rate by weight, it will also be discharged at a uniform rate by volume so long as its bulk density remains unchanged, and since the conveyor is moving at a constant speed, the deposit of a uniform volume of coal thereon per unit of time will be reflected by a uniform depth of coal on the conveyor. If the depth of coal on the conveyor increases, it means that the same weight of coal now occupies a larger volume than it did before, i. e., its bulk density has decreased. Similarly, if the depth of coal on the conveyor decreases, it means that the same weight of coal now occupies a smaller volume, i. e., its bulk density has increased. Accordingly, changes in the depth of coal on the conveyor may be used to measure changes in the bulk density of the coal that is introduced into the hammer mill from the blending bins.

To control the bulk density of the coal within desired limits, and to maintain at the same time a minimum moisture content, means are provided for adding oil and/or Water to the coal stream. Those additions are preferably made before the coal is pulverized, so that the oil and/ or water will be intimately mixed with the coal while it is being reduced in the hammer mill. For this purpose, a water pipe 21 and an oil pipe 22 are mounted with their outlets 23 and 24, respectively, above the inlet of the hammer mill, and the other ends of those pipes are respectively connected to a water storage tank 25 and an oil storage tank 26. Valve means 31 and 32 are provided for regulating the flow of liquid through those pipes. A depth gauge 35, which is pivotally mounted to swing in a vertical plane above the conveyor 15, is set at a predetermined height that is slightly above the average level of the top of the coal bed in the conveyor when the coal thereon has the desired minimum moisture content and the desired bulk density. Variations in the average level of the coal on the conveyor, corresponding to changes in its bulk density, can then be visually observed and controlled Within desired limits by adding the necessary amounts of oil and/ or water. For example, Whenever the average depth of pulverized coal on the conveyor is lower than the predetermined height indicated by the gauge, the coal has a higher bulk density (lower moisture, content) than is desired and that condition can be remedied by opening valve 3-1 in the water line and adding the necessary amount of water to the coal in the hammer mill. Similarly, if the depth of coal on the conveyor rises above a predetermined level, indicating too low a bulk density, the addition of oil to the coal by opening valve 32 will restore the desired bulk density. Occasionally, it may be desirable to add a minimum amount of oil to the coal and control its bulk density by adding or withholding water.

Bulk density control of coking coal in accordance with the present invention has been successfully practiced by applicants with a blend of four different coals having a varying moisture content. The types of coals used and the quantities of each were as follows:

The above types of coal were delivered at the rates indicated to a hammer mill, where they were blended together and reduced in size to particles ranging from about 1 /2 inches to dust (1 /2 X of which about 75% would pass through a /8 inch screen. Over a given period, the surface moisture content of the above coal after it was blended and pulverized was found a vary considerably, being on an average somewhat less than the 4 /2% considered best for the particular coking operation. To compensate for the decrease in its bulk density due to moisture in excess of 4 /2 and also due to its reduction in the hammer mill (lump coal having a greater bulk density than the same coal after it is pulverized), it was found practicable to add oil to the coal in the hammer mill at the rate of about 25 gallons per hour. Variations in the bulk density of the coal due to variations in its surface moisture content, where the moisture did not exceed 4%%, were then controlled by adding varying amounts of water at a rate up to 750 gallons per hour. In other words, with this particular blend of coals, the bulk density was largely controlled by adding varying amounts of water, plus the addition of uniform amounts of oil, changes in the latter being made only when the surface moisture content exceeded certain limits. In determining the amount of oil and/or water to be added to the coal, the delivery conveyor onto which the coal was discharged from the hammer mill was run at a uniform speed suificient to maintain the top of the coal thereon at an average height of about six inches above the edge of the conveyor when that coal was of the proper bulk density and moisture content. That height was maintained within limits of about :Mr inch by observing the clearance between the height gauge and the top of the coal stream. When the height increased more than inch above the desired level, the operator reduced the amount of water added to the coal. When cutting on the water entirely did not sufliciently reduce the height of coal, the operator increased the amount of added oil to bring the height of the coal back to the desired level. Conversely, when the height of the top of the coal stream dropped more than A inch below the desired level, the

operator increased the amount of water added to the coal until its height was raised the necessary amount. If the bulk density had not been controlled in this way, it might have varied as much as 16%, and the height of the coal stream might have varied over a range of one inch. However, by adding oil and/ or water to the coal as indicated by changes in the height of the coal shown on the conveyor, the bulk density of the coal charged into the coke ovens was maintained substantially constant and the coal was assured the minimum moisture content that was most beneficial for the formation of coke oven gas.

It is an adxantage of this invention that it provides an economical method of controlling the bulk density of coking coal within narrow limits, and one that can be easily used in existing plants without expensive alterations to existing equipment. 7 V

While it has been found satisfactory to control the bulk density of coking coal by visual observations of its height on a moving conveyor and by manually regulating oil and/or water additions to the coal, it will, of course, be apparent that the same operation could be performed automatically by means of well known and readily available indicating and regulating devices.

' We claim:

1. The method of obtaining coking coal of uniform bulk density that includes the following steps: discharging coal at a uniform rate by weight onto a conveyor,

' moving the conveyor at a uniform speed whereby the height of the coal thereon will be uniform so long as its bulk density remains uniform and will vary inversely with changes in its bulk density, and maintaining the height of coal on the conveyor at a uniform predetermined level corresponding to the desired bulk density by adding oil to the coal before it is discharged onto the conveyor to decrease the height of coal on the conveyor to said predetermined level and by adding water to the coal before it is discharged 'onto the conveyor to increase the height of coal on the conveyor to said predetermined level, thereby to maintain uniform the bulk density of the coal on the conveyor.

2. The method according to claim 1 that includes the additional step of pulverizing the coal before it is discharged onto the conveyor.

7 3. The method according to claim 1 that includes the following additional steps: feeding coal at a uniform rate by weight to a pulverizer and pulverizing the coal at a uniform rate before discharging it onto the conveyor.

4. Apparatus for controlling the bulk density of coking coal that is a blend of different types of coal, comprising a blending bin for holding each type of coal, a weight controlled feeder associated with each bin for delivering coal from the bin at a predetermined uniform rate by weight, a hammer mill for mixing the coal delivered by the feeders and for reducing the coal in size at a uniform rate, a conveyor for receiving coal at a uniform rate from the hammer mill, means for moving the conveyor at a uniform speed whereby the height of coal thereon will be uniform so long as its bulk density remains uniform and will vary inversely with changes in its bulk density, gauge means for visually determining variations from a predetermined level in the height of coal on the conveyor, and manually operable means for separately adding oil and water to the coal before it is discharged from the hammer mill to maintain the coal on the conveyor at said predetermined level and thereby obtain coal of uniform bulk density.

References Cited in the file of this patent FOREIGN PATENTS 620,133 Great Britain Mar. 21, 1949 

1. THE METHOD OF OBTAINING COKING COAL OF UNIFORM BULK DENSITY THAT INCLUDES THE FOLLOWING STEPS: DISCHARGING COAL AT A UNIFORM RATE BY WEIGHT ONTO A CONVERYOR, MOVING THE CONVEYOR AT A UNIFORM SPEED WHEREBY THE HEIGHT OF THE COAL THEREON WILL BE UNIFORM SO LONG AS ITS BULK DENSITY REMAINS UNIFORM AND WILL VARY INVERSELY WITH CHANGES IN ITS BULK DENSITY, AND MAINTAINING THE HEIGHT OF COAL ON THE CONVEYOR AT A UNIFORM PREDETERMINED LEVEL CORRESPONDING TO THE DESIRED BULK DENSITY BY ADDING OIL TO THE COAL BEFORE IT IS DISCHARGED ONTO THE CONVEYOR TO DECREASE THE HEIGHT OF COAL ON THE CONVEYOR TO SAID PREDETERMINED LEVEL AND BY ADDING WATER TO THE COAL BEFORE IT IS DISCHARGED ONTO THE CONVEYOR TO INCREASE THE HEIGHT OF COAL ON TO THE CONVEYOR TO SAID PREDETERMINED LEVEL, THEREBY TO MAINTAIN UNIFORM THE BULK DENSITY OF THE COAL ON THE CONVEYOR. 